1. Field of the Invention
The present invention relates an exhaust gas purifying system for an internal combustion engine and particularly, to an exhaust gas purifying system for an internal combustion engine, which can purify hydrocarbons (referred to as HC hereinafter) with a lower electric power.
2. Description of the Related Art
A variety of studies for purifying exhaust gases from an internal combustion engine e.g. of an automotive vehicle have been developed to prevent pollution of the atmosphere. Known harmful substances in the exhaust gas from the vehicle are HC, CO, and NOx. For example, one of schemes for purifying HC is disclosed in Japanese Patent Laid-open Publication Hei 5-10117.
As depicted in the Publication, the scheme incorporates a combination of a catalyst converter with a heater function and a catalyst converter with no heater function in order to minimize the emission at the start of the engine. According to the scheme, the catalyst converter with the heater function quickly heats up a catalyst to a temperature of activation level. Hence, the emission at the start of the engine will be minimized.
Another scheme is disclosed in Japanese Patent Laid-open Publication Hei 5-31359. The scheme has a zeolite adsorbent, a honeycomb heater, and a main monolithic catalyst provided in an exhaust gas passage of an automotive vehicle so that HC in the exhaust gas of a lower temperature at the start of an engine is adsorbed by the adsorbing action of the zeolite adsorbent and further purified through energizing the honeycomb heater to instantaneously activate the catalyst on the heater.
However, quickly heating up the catalyst with the heater requires a large amount of electric power in a short time. This allows a limited volume of the catalyst to be heated up. Since the exhaust gas purifying system is designed for the internal combustion engine of an automotive vehicle, its power consumption depends on a particular source such as a battery or an alternator. Accordingly, the higher power consumption demands a more supply of electric power from the source and will cause the source to be increased in the capacity.
The scheme also has not proposed to purify according to type of HC desorbed from the adsorbent-catalyst.